1. Field of the Invention
The invention relates to a system for dampening the movement of a floating structure, for instance a semi submersible platform, where the floating platform comprises a deck structure, several columns which support the deck structure and penetrate the water plane surface and a pontoon structure and which floating structure is moored to the seabed.
2. Discussion of Related Art
There are several reasons why a floating structure should have as little movement or as reduced heave pitch and roll movement as possible. Especially for floating structures as semi submersible platforms or wellhead platforms are there a strong need to reduce the motion. A wellhead platform normally is understood to be a platform in an oil/gas offshore installation complex which accommodate the production risers of the production streams, preferably with dry wellheads and some process equipment for oil/gas treatment, but normally not full production package for treatment of the hydrocarbons to final export specifications. There will also be export risers, which can be hard risers, flexible risers or steel catenary risers (SCR), since there is limited storage capability in a semi submersible platform.
In order to accommodate dry wellheads, the motion characteristics of the unit must be favourable to reduce tensioner stroke requirements and extra loads on the risers. Traditionally, wellhead platforms have been jackets for small drafts of water and tension leg platforms (TLPs) for larger drafts. Jackets become unsuitable for drafts in excess of a couple of hundred meters. TLP platforms will be expensive and difficult to install for drafts in excess of about 1000 meters. Especially the tethers and the foundations of the TLPs are increasingly complex for the larger drafts.
An ordinary semi submersible platform will not possess the motion characteristics required to allow dry wellheads in most required environments. Such a platform must therefore get special characteristics to reduce the motions to a level that can be acceptable for dry wellhead and their riser tension compensators
There have been several attempts to reduce the motion characteristics of a floating structure as a semi submersible platform. Some of these techniques are mentioned below.
In U.S. Pat. No. 4,829,928 and ocean platform has a negative buoyant pontoon suspended from the balance of the platform to increase the heave resonant period to at least 25 seconds. Tendons suspend the pontoon to a depth where dynamic wave forces do not materially act directly on it in seas of normally occurring periods of up to about 15 seconds. Columns and upper pontoon provide buoyancy for the platform.
EP 359 702 describes a semi submersible platform which includes a fully submerged lower hull and a plurality of stabilizing columns which extend from the lower hull to an upper hull. Each column has a dynamic wave zone in the design seaway. At least one column has means adapted to reduce its water plane area to increase the natural heave period of the platform, so that the natural heave period becomes grater than the longest period of any wave with substantial energy in the design seaway, thereby lowering the platform's heave response. These means are a channel which in use becomes flooded with sea water.
U.S. Pat. No. 6,431,107 describes a floating offshore structure with a buoyant hull with sufficient fixed ballast to place the centre of gravity of the floating structure below the centre of buoyancy of the hull. A support structure coupled to an upper end of the hull supports and elevates a superstructure above the water surface. A soft tendon is attached between the hull and the seafloor.
This floating structure has as a consequence a very high natural period during heave, and due to very little restoring force it may have large second orders heave motions. To reduce the second order heave motion the platform is provided with a vertical anchoring system with a vertical stiffness as large as possible but limited so that the resulting natural period during heave is at least 20 second.